1. Field of the Invention
This invention relates generally to a light measurement device. More specifically this invention relates to a light measurement device attuned to the visible spectrum. Even more specifically this device relates to a sunlight integration instrument whose output is accumulated light over a given time period.
2. Problems in the Art
Photosynthesis is an important biochemical process in which algae, plants, and some bacteria convert the energy of sunlight to chemical energy. The chemical energy is used to drive reactions such as the formation of sugars or the fixation of nitrogen into amino acids. Generally speaking, photosynthesis is controlled by four factors, (1) temperature, (2) CO2 concentration, (3) light irradiance, and (4) light wavelength. Unless plants are grown in controlled environments, man has little control over temperature and CO2 concentration. However, in many instances man has the ability to plant crops and cultivate gardens in areas that receive the most direct sunlight, thus has some control over the irradiance and wavelength variables.
Irradiance is a radiometric term for the power of electromagnetic radiation at a surface, per unit area or, more specifically, when power is incident on the surface. The SI units for irradiance is watts per square meter (W·m−2). Irradiance is sometimes called intensity, but this usage can lead to confusion with radiant intensity, which has different units. Table 1 depicts common radiometric units.
TABLE 1Radiometric Units - SIIrradiancewatt per square meterW · m−2Radiancewatt per steradian perW · sr−1 · m−2square meterRadiant emittance/watt per square meterW · m−2Radiant exitanceRadiant energyjouleJRadiant fluxwattWRadiant intensitywatt per steradianW · sr−1Spectral irradiancewatt per meter3 orW · m−3 orwatt per square meterW · m−2 · Hz−1per HertzSpectral radiancewatt per steradianW · sr−1 · m−3 orper meter3 orW · sr−1 · m−2 · Hz−1watt per steradianper square meterper Hertz
All of these radiometric quantities characterize the total amount of radiation present, at all frequencies. It is also common to consider each frequency in the spectrum separately. When this is done for radiation incident on a surface, it is called spectral irradiance, and has SI units W·m−3, or commonly W·m−2·nm−1. If a point source radiates light uniformly in all directions and there is no absorption, then the irradiance drops off in proportion to the distance from the object squared, since the total power is constant and it is spread over an area that increases with the square of the distance from the source.
The accurate measurement of available sunlight is difficult. Judgment based on visual observation is completely inadequate since the human eye, which accommodates a brightness range varying in intensity over a factor of 100,000 or more, ordinarily cannot discern changes of a factor of two or three or four. But a factor of two might be critical for the health of a plant. Furthermore, research has shown that identical plants, placed as little as ten feet apart in visually identical conditions, can experience markedly different growth rates where the only differing environmental variable that could be detected was a minor difference in useful light that resulted in differences in irradiance on the surface of the plants at both locations. It is clear that a sensitive light measurement instrument is required to evaluate light conditions to optimize photosynthetic promoted growth in plants.
The measurement of light intensity, or quantification of light received, by an instrument at any given moment is a well known process involving the use of a light meter or lux meter. Several systems of light meters have been used, the three most common being selenium, CdS, and silicon light meters.
Selenium and silicon light meters use sensors that are photovoltaic. These sensors generate a voltage proportional to light exposure. Selenium sensors generate enough voltage for direct connection to a meter. Silicon sensors need an amplification circuit and require a power source like a batteries to operate. CdS light meters use a sensor based on photoresistance. These also require a battery to operate. Most modern light meters use silicon or CdS sensors. They indicate the exposure either with a needle galvanometer, or on a LCD screen. Typical uses are to determine light intensity for photographic applications such as determining proper exposure. Given a film speed and shutter speed, the meter will show the f-stop which would give a neutral exposure of an ideal gray slate. These instruments have also been adapted to measuring the light intensity for agricultural or gardening applications.
Typically, the measured light covers the entire range of wavelengths for which the various kinds of chlorophyll (and other photosynthetic compounds) in plants are effective in converting light to plant energy. With a bit of arbitrariness, this has been defined as wavelengths over the range of 400 to 700 nanometers. This is roughly the same range that a human eye can perceive—i.e., colors from violet, then blue, green, yellow, orange and finally red. Eye sensitivity peaks in the center of this range (green, about 500-550 nanometers) and falls off to almost zero at the extremes, while the typical light sensor has uniform sensitivity over that wavelength range (or light bandwidth).
The spectrum available for photosynthesis is limited to visible light because infrared light does not contain enough energy for photosynthesis and ultraviolet light has too much energy. Ultraviolet light intercepted by plants can create free radicals, which can break chemical bonds in an organism. The wavelength and quantity of light is also important in other biological processes such as the inhibition of hypocotyl elongation. The hypocotyl is the primary organ of extension of a young plant and develops into the stem.
The light reactions of photosynthesis (the reactions that convert light energy to chemical energy in the form of ATP and NADPH and produce O2 as a by-product) occur in the thylakoid membranes of the grana contained within chloroplasts, primarily in palisade mesophyll tissue of terrestrial plant leaves. Thylakoid membranes contain several protein pigments (phytochromes), including chlorophyll α, chlorophyll β, and carotenoids. Chlorophyll α participates directly in the light reactions of photosynthesis. It absorbs mainly blue-violet and red light, and reflects green light. This correlates well with overall photosynthetic efficiency, indicating that this molecule is the most important to the process of photosynthesis. However, energy absorbed by chlorophyll α is not the only energy that can be used in photosynthesis. Chlorophyll β is very similar in structure to chlorophyll α, but it absorbs mainly blue and orange light, reflecting yellow-green. It then transfers the collected energy to chlorophyll α, which actually utilizes it in the light reactions. Some carotenoids can also transfer collected energy to chlorophyll α. The absorption spectra of chlorophyll α and chlorophyll β are shown on graph below. Peaks of the graph indicate high rates of absorption; troughs are low rates. Chlorophyll absorption efficiency correlates closely with overall photosynthetic efficiency, indicating that chlorophyll is the main photosynthetic pigment. Carotenoids such as xanthophylls, anthocyanins, and carotenes absorb mainly blue-green light, reflecting, thus, yellow-orange. Some carotenoids act like chlorophyll β in passing their energy on to chlorophyll α while others help to diffuse excess light energy, which could denature the chlorophyll.
Photons contain varying amounts of energy based on their wavelength; shorter wavelengths contain more energy, therefore blue wavelengths are very high-energy, one reason why they are absorbed by all three photosynthetic pigments and are highly efficient at powering photosynthesis. When a pigment absorbs a photon of light energy, one of its electrons gains energy. It is very unstable, and soon falls back to its ground state, releasing heat and/or light along the way. In photosynthesis, however, excited electrons are donated to a primary electron acceptor before they revert to their former state.
Uniform sensitivity in a light meter over the visible spectrum can give non-ideal results, since plants generally don't use light at wavelengths in the center of that range, thus revealing why plant leaves appear green—green light is reflected by leaves, while other colors of light are absorbed and used by plants. Under a forest canopy, more green light is present than would be “out in the open” because of the preferential absorption of other colors, and therefore an “ideal” light sensor gives too high a reading—it is reporting all that green light which isn't used by plants. This becomes extremely complicated when one takes into account the fact that not all plants absorb light in the same way or of the same wavelengths. Thus additional wavelengths may need to be filtered out as well to achieve accurate results.
Dual sensor systems are described by U.S. Pat. No. 3,746,430 to Brean et al., U.S. Pat. No. 4,580,875 to Bechtel et al., U.S. Pat. No. 4,793,690 to Gahan et al., U.S. Pat. No. 4,886,960 to Molyneux et al., U.S. Pat. No. 4,917,477 to Bechtel et al., U.S. Pat. No. 5,204,778 to Bechtel, U.S. Pat No. 5,451,822 to Bechtel et al., U.S. Pat. No. 5,715,093 to Shierbeek et al., U.S. Pat. No. 6,504,143 to Nixon et al., U.S. Pat. No. 6,359,274 to Nixon et al., and U.S. Pat. No. 6,737,629 to Nixon et al., each of which is incorporated herein by reference.
The use of dual sensor photodiodes as described by the patents by Nixon et al. as set forth in the previous paragraph use one photodiode to generate a charge in proportion to the amount of incident light. The device measures incident light across the visible spectrum. The resulting potential reflects the amount of incident light accumulated at the sensor over a period of time thus providing a measure of light over the accumulation period without resorting to the necessity of a mathematical integration process. One key component of the dual sensor technologies is the ability to measure noise with a second, shielded sensor and thereby improving the accuracy, precision and sensitivity of the device.
The Nixon et al. patents use logic to determine the length of time over which the photodiode and accompanying circuitry will collect the charge generated by light striking the photodiode. The circuitry is reset and a charge is collected over a predetermined time period. The amount of charge collected over that time period determines the length of an electrical pulse generated by the circuitry and the length of this pulse is the signal parameter that indicates the quantity of light striking the photodiode over that time period.
A particular method that is used is to couple the photodiode with a capacitor via a switch. The switch between the photodiode closes and the capacitor is charged up to a fixed voltage. Once the predetermined time period has passed, the switch changes and the charge on the capacitor is dissipated at a fixed rate. The length of the discharge determines the length of the pulse which indicates the intensity of the light or glare.
U.S. Pat. No. 4,249,109 by Ogawa discloses and claims a light digitizing circuit for an electronic flash drive. Ogawa uses a light sensitive sensor to develop a current upon being exposed to light. In particular, it is intended to measure the light from a flash and stop the flash once a predetermined quanity of light has reached the aperture. The light sensor in at least one embodiment is a photodiode which creates a current when exposed to light. This current charges a capacitor, and when the capacitor reaches a predetermined voltage, it is discharged. A counter keeps track of how many times the capacitor is charged and discharged, and thereby a measurement of light exposure is achieved. Each time the capacitor is charged represents a quantity of light. More intense light will charge the capacitor faster, and less intense light will charge the capacitor more slowly, but the quantity of light having reached the photodiode is still indicated by the total charge on the capacitor.
U.S. Pat. No. 5,583,605 by Sakamoto claims and discloses a photoelectric current converting circuit. Sakamoto is directed to a control circuit for adjusting an electronic flash on a camera. The circuit generally comprises a capacitor, a voltage supply circuit, a comparator, an outputting circuit, a discharging circuit, and a plurality of current sources and a selecting member to select one of the current sources. The current source associated with this circuit is, in the preferred embodiment, a photodiode. The electric current generated by the photodiode is output to a current converting circuit which uses a capacitor and a comparator to generate a string of logic level voltage pulses. The voltage pulses are counted by a counter circuit to integrate the current generated by the photodiode and the output from the counter circuit is compared with the output from the standard value circuit. When the value of the counter output equals or exceeds the standard from the standard value circuit, the circuit generates a stop signal to the light emitting circuit so that, in this case, the flash for a camera is stopped.